Photodichroic crystals and preparation thereof

ABSTRACT

The improvement of the electrolytic and additive methods of coloring alkali fluoride crystals comprising the step of selecting an alkali fluoride crystal having the impurities of between 0 and 5 ppmA of OH; between 0 and 50 ppmA of one or more alkaline earth cations, of which between 0 and 10 ppmA Mg ++  and between 0 and 10 ppmA Ca ++  ; between 0 and 50 ppmA of non-fluorine halide anions; between 0 and 10 ppmA of SiF 6   =  ; between 0 and 30 ppmA of other ions; and the method of preparing these crystals which comprises converting the alkali fluoride to alkali hydrofluoride, crystallizing and vacuum drying the alkali hydrofluoride crystals, regenerating the alkali fluoride by expelling hydrogen fluoride, and crystallizing the alkali fluoride.

BACKGROUND OF THE INVENTION

The invention relates generally to a method for preparing photodichroicalkali fluoride crystals and more particularly to additive andelectrolytic coloration of alkali fluoride crystals.

Photodichroism is the property of absorbing wavelengths of lightdifferently along each axis of the crystal. To impart this property in acrystal, the crystal is colored, i.e., has color centers introduced intoits crystalline structure. The most common of these centers are the F,F_(A), M, and M_(A) centers with the M-center being the most important.

Alkali halide crystals containing color centers such as theaforementioned ones have shown considerable promise as optical elementscapable of reversible, on-line write-erase-read operations where thereading would be nondestructive. Information would be related to thepresence or absence of dichroic absorption (i.e. -- differences inabsorption of polarized light) and would be introduced into the crystalby causing M-center reorientations with polarized light.

The most promising alkali halide for possible use near room temperatureis NaF. Unfortunately, this fluoride, like the other alkali fluorides,has been colored only through the use of ionizing radiation such asX-rays or high-energy electrons. But such a technique introducesinterstitials which eventually recombine with the color centers andmutually annihilate. The crystals accordingly are less stable thencrystals colored by other techniques.

Two methods of coloring alkali halide crystals which do not produceinterstitials are the additive and electrolytic methods. However,attempts to produce well-characterized color centers in alkali fluoridecrystals by these techniques have not been successful. The crystals areeither not colored or contain metallic-colored particles with diametersas large as 30 nm. Such particles give rise to Tyndall scattering withan apparent absorption in the wavelength range from 505 to 570 nm.

It has been known that alkali earth metals cations cause metallicparticles to form during coloration. Previous methods were highlysuccessful in removing these cations along with other foreign cationsexcept for Mg⁺⁺ and Ca⁺⁺. Also these methods of purifying alkalifluorides were not able to eliminate the fluorosilicate ion (SiF₆ ⁼)below 10 ppmA and were extremely slow and inconsistent in removing thehydroxyl ion.

Throughout this disclosure concentrations are expressed in units ofppmA. Such units are synonymous with the units of moles of additive per1,000,000 moles of crystal.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to produce alkali fluoridecrystals with well characterized color centers by an additive or by anelectrolytic method.

Another object of this invention is to provide a method of consistentlypreparing alkali fluoride crystals with a maximum hydroxyl ionconcentration of 0.5 ppmA.

Another object of this invention is to provide a method of preparingalkali fluoride crystals with a maximum hydroxyl ion concentration of0.5 ppmA in less than 3 days.

And another object of this invention is to provide alkali fluoridecrystals with a maximum hydroxyl ion concentration of 5 ppmA.

And still another object of this invention is to provide alkali fluoridecrystals with a maximum hydroxy concentration of 0.5 ppmA.

And yet another object of this invention is to provide an alkalifluoride crystal with color centers having greater stability.

A further object of this invention is to provide alkali fluoridecrystals with color centers having less fatigue in switchingpolarization directions.

A still further object of this invention is to provide colored alkalifluoride crystals with greater purity in their absorption spectra.

These and other objects are achieved with an alkali fluoride crystalhaving impurities of between 0 and 5 ppmA of OH⁻ ; between 0 and 50 ppmAof one or more alkaline earth cations, of which between 0 and 10 ppmAMg⁺⁺ and between 0 and 10 ppmA Ca⁺⁺ ; between 0 and 50 ppmA ofnon-fluorine halide anions; between 0 and 10 ppmA of SiF₆ ⁼ ; between 0and 30 ppmA of other ions; and the method of preparation which comprisesconverting the alkali fluoride to alkali hydrofluoride, crystallizingthe alkali hydrofluoride, vacuum drying these crystals, regeneratingalkali fluoride from the alkali hydrofluoride by expelling gaseous HF ina heated stream of an inert gas, melting the NaF in an inert atmosphere,withdrawing up to 90% of the melt into a single-crystal boule by theCzochralski technique, and repeating the above melting and withdrawingsteps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 compares the absorption spectra of additively colored researchgrade commercially obtained NaF and of additively colored NaF of thisinvention.

FIG. 2 compares the absorption spectra of the commercially obtainedcrystals of FIG. 1 and of electrolytically colored NaF crystals of thisinvention.

FIG. 3 compares the absorption date of high purity commercially obtainedKF and of KF of this invention, both have been additively colored withK.

FIG. 4 graphically shows the absorption spectra of KF of this invention,doped with NaF additively colored with K and aggregated.

FIG. 5 graphically shows the absorption spectra of KF of this invention,doped with LiF, additively colored with K and aggregated.

DETAILED DESCRIPTION OF THE INVENTION

It is not fully understood why the trace presence of OH⁻ with alkalineearth cations is detrimental to the coloration of alkali fluoridecrystals by the additive and electrolytic methods. However, since theions complex easily and the complexes are considerably larger than thefluoride ion, the mobilities of these complexes are considerably lessthan that of the fluoride. Thus it is believed that the complexes impedethe movement of fluoride ions through the crystal lattice which is basicto the mechanism of both the additive and electrolytic methods ofcoloration. This is only a possible explanation and is not meant to bindthe present invention to any explanation. Although the exact mechanismis not fully understood, it is now evident that the hydroxyl ion has anenormous influence on the coloration of alkali fluoride crystals andthat exceptional colored alkali fluoride crystals are now possible.

In the practice of the instant invention, alkali fluoride is reactedwith an aqueous hydrogen fluoride solution at a temperature from about25° C to about 90° C with 80° C to 90° C preferred to form an alkalihydrofluoride. The purity of the alkali fluoride is not critical. Due toconsiderations of processing time and cost, the CP grade is preferred.Likewise, great latitude is possible with the concentration of theaqueous HF solution, but for processing considerations the concentrationshould preferably be above 20 percent by weight. The effect of usingexcessively strong acid, e.g. 40% by weight, is the production ofpolyhydrofluorides. Such alkali polyhydrofluorides do not present anyproblems except to add additional heating steps to the alkali fluorideregeneration procedure. This aspect is discussed in detail later.

If the hydrofluoride reaction was carried out at the preferredtemperature range, the alkali hydrofluoride is crystallized by reducingthe temperature of the solution to room temperature. A slow reduction oftemperature (on the order of 10 to 12 hours) is preferred because of thelarger resulting crystals. If the hydrofluoridation reaction was carriedout at a lower temperature than 80° C, the temperature of the solutionis increased to 80° C to 90° C and additional precipitated hydrofluorideis dissolved in the hot solution. Upon equilibration, the solution isdecanted and slowly cooled as before.

The crystals are separated from the liquid. One possible procedurecomprises draining the excess liquid off and vacuum drying the crystalsat a vacuum of 10 torr or less for about 12 to about 24 hours. Since itis critical that the crystals contain no water, as much water should beremoved at this stage as possible.

The regeneration of alkali fluoride from the hydrofluoride crystals isan extremely important technique of the method of this invention. It hasbeen determined that bubbling of the hydrofluoride, caused by heatingthe crystals too rapidly, adversely affects the yield of alkalifluoride. Further the alkali fluoride being generated must remain moltenand be allowed to consolidate and free itself of bubbles for at least11/2 hours in order to minimize water recontamination.

The regeneration of the alkali fluoride has to occur in an inert andnoncontaminating atmosphere. Examples of gases which may be used areargon and helium. Exemplary of possible getters for the gases aretitanium, barium, and calcium. The atmosphere most often used isTi-gettered argon. Similarly the equipment in contact with the crystalsmust be inert and noncontaminating. Accordingly, such equipment must beconstructed from or coated with platinum, fluorinated polymers, glassyor other forms of pure carbon, and the like. The hydrogen fluoride iscarried away by a suitably heated stream of an inert gas.

Alkali polyhydrofluorides lose hydrogen fluoride at a lower temperaturethan alkali monohydrofluorides. If such polyhydrofluorides are present,the crystals are initially heated to a temperature from about 80° C toabout 90° C, taking at least 2 hours to reach this temperature. Thetemperature is then maintained for at least one hour thereby eliminatingwater. In order to eliminate any unreacted hydrogen fluoride, thetemperature is raised in a time not less than 3 hours to a temperaturefrom about 100° C to about 3/4 of the Centigrade temperature at which HFgas starts to evolve from the crystals as determined from the phasediagram. This temperature is held for at least one hour.

The temperature is again raised over a period of time not less than 5hours and preferably of about 6 hours to the temperature at which thepolyhydrofluoride again begins to evolve HF and is held at thattemperature for about 2 hours. This progression is continued up to thetemperature at which the alkali monohydrofluoride evolves a pressure of760 torr of hydrogen fluoride. That temperature is held for at least 4hours and preferably for 6 hours in order to ensure the completeconversion to alkali fluoride.

If the initial crystals are mostly, i.e., 95%, the monohydrofluoride,the temperature is raised over a period of time not less than 5 hoursand preferably to 6 hours above 100° C but not higher than three fourthsof the Centigrade temperature at which the monohydrofluoride begins toevolve HF. This heating serves to expell unreacted HF and water. Thetemperature is raised over a period of time of at least 5 hours andpreferably of 6 hours to the temperature at which a pressure of 760 torrof HF is evolved from the crystals as determined by the phase diagram.Heating is continued at this temperature for at least 4 hours andpreferably for 6 hours.

At this point the regeneration procedure is the same regardless ofwhether the crystals were initially a mixture of mono andpolyhydrofluorides or predominantly the mono form. The temperature isagain increased over a period of time of at least 6 hours and preferablyof 12 hours to a temperature of about one half of the melting point ofthe particular alkali fluoride in degrees Centigrade. The temperature isheld for about one hour before the temperature is increased to themelting point of the alkali fluoride. This latest temperature increasetakes at least 2 hours and preferably takes 4 hours and the temperatureis held for at least 2 hours. This last heating is to ensure that no HFis present in the molten alkali fluoride.

The melt, having been freed of water and HF, is crystallized into a barin the following manner. The temperature is reduced by 100° C to 200° Cover a period of time of at least 4 hours and preferably of 6 hours.With this temperature reduction a polycrystalline bar begins to form.The bar is completed as the temperature is further reduced to roomtemperature over a period of time of at least 4 hours.

The crystallization procedure further comprises withdrawing up to 75-90% of the melt into a single-crystal boule preferably by the Czochralskitechnique.

The Czochralski technique was first published by Czochralski J., in Z.Physik Chem. 93, 219-221 (1918) and is herein incorporated by reference.Other crystallization techniques may be used in the practice of thisinvention.

To illustrate the purification method of this invention the followingexample is presented. It is understood that the example and all theexamples of the disclosure are given by way of illustration and notmeant to limit in any manner the specification or the claims to follow.

EXAMPLE I

Reagent-grade (Baker and Adamson) NaF was reacted with a 40% aqueoussolution of hydrofluoric acid (HF) at a temperature of 90° C in aplatinum beaker. This reaction gave almost a 100% yield of themonohydrofluoride. The NaHF₂ was crystallized by reducing thetemperature to room temperature (20° C) in 12 hours in a platinumbeaker. The liquid was drained from the platinum beaker and the crystalswere vacuum dried at a vacuum of 10 torr for 18 hours in a teflonevaporation disk. Regenerating NaF comprised heating in a glassy carbonboot in a horizontal tube furnace the crystals to 150° C in 6 hours,maintaining 150° C for 2 hours, heating to 225° C in 6 hours, holding225° C for 6 hours, heating to 500° C in 12 hours, holding 500° C for 1hour, heating to 1000° C in 4 hours, holding for 2 hours, andfurnace-cooling to room temperature. The NaF was remelted in a platinumcrucible and was crystallized by withdrawing 80% of the melt into asingle-crystal boule by the Czochralski technique, then reducing thetemperature to room temperature in 2 hours. The middle 90% of thecrystal was remelted and regrown by a repetition of the Czochralskimethod, into a second-generation single-crystal boule.

The second-generation NaF crystals were found by sparksource massspectrography to contain 10- 30 ppmA of chlorine, 1- 10 ppmA of calcium,and about 2 ppmA of potassium. Absorption measurements at 0.151 and 2.68μm revealed the presence of 0.05 - 0.5 ppmA of OH⁻ in undoped specimensand about 1 ppmA of OH⁻ in Li-doped crystals.

The alkali fluoride crystals of this invention can be colored by anyadditive or electrolytic technique. As was expected, the heavier alkalifluorides such as potassium fluoride and rubidium fluoride were theeasiest and quickest to color. The two heaviest alkali fluorides, i.e.,cesium and francium do not have a crystalline structure suitable for anypractical use of their colored centers.

The following two examples illustrate an additive coloration and anelectrolytic coloration of NaF crystal prepared by the method of ExampleI.

EXAMPLE II

For additive coloration, the crystals and about one-tenth their weightof Na metal were sealed at room temperature under 0.5 atm. of argon in avacuum-annealed stainless-steel tube. The temperature is raised to 900°C and is maintained at 900° C for 16 hours. Then the crystals are cooledin 4 hours to room temperature.

EXAMPLE III

For electrolytic coloration, crystals were held in slowly flowing heliumbetween a pointed tungsten electrode and a flat graphite anode, with anelectric field strength of 100V/cm applied.

The next examples illustrate the additive coloration of this invention.The crystals had a hydroxyl ion concentration of 2 ppmA.

EXAMPLE IV

KF crystals and about one-tenth of their weight of K metal were sealedat room temperature under 0.5 atm. of argon in a vacuum-annealedstainless-steel tube. The temperature was raised to 700° C and held for16 hours. Then the temperature was reduced to room temperature in 4hours.

The absorption spectra of the crystals colored in Example II weredetermined. The results are shown and compared with commerciallyavailable NaF crystals in FIG. 1. Curve A represents the absorption dataof a research grade commercially available crystal which was prepared bythe "Cornell" method and additively colored by a method similar to themethod of Example II. The color centers in a slice were dispersed byreheating to 900° C for one minute and by cooling to room temperature in5 minutes. Curve B represents the absorption data of a slice of the samecommercial crystal. It was reheated in argon to 960° C for one minuteand then quenched (5 sec.) to room temperature (20° C). Curve Crepresents one of the crystals of Example II which also had a postcoloring anneal of 880° C followed by a quench of 10 sec. to roomtemperature.

As can be seen in FIG. 1, NaF crystals of this invention give a trueM-center as the crystals of Curves A and B do not. Further the quantityof F-centers is three times greater than the best of the commercial NaFcrystals.

The same determinations were made for crystals of Example III and acomparison similar to FIG. 1 was made in FIG. 2. Curves A and Brepresent the additively colored commercial crystals and Curve C is thedata from a crystal of Example III with a post coloring anneal. Againthe same exceptional results were obtained.

FIG. 3 graphically compares the absorption spectra of research gradecommercially obtained KF prepared by the "Cornell" method and of KF ofthis invention. Curve A represents the commercial KF and Curve Brepresents the KF of this invention.

The alkali fluoride crystals of this invention may be doped in additionto being colored in order to form the F_(A) and M_(A) centers in thecrystals upon aggregation. FIGS. 4 and 5 graphically show the absorptionspectra of KF crystals doped with NaF and LiF.

As can be clearly seen from the figures exceptionally colored KFcrystals are possible if the crystals selected do have a hydroxyl ionconcentration between 0 and 5 ppmA, a SiF₆ ⁼ between 0 and 10 ppmA,Mg⁺ + concentration between 0 and 10 ppmA, and Ca⁺⁺ concentrationbetween 0 and 10 ppmA. Such crystals have only been obtainedconsistently and conveniently with the method of the present invention.This method has without failure produced the important NaF and KFcrystals with the aforedescribed purity in less than three days. Alltemperatures in the specification and in the claims to follow are indegrees Centigrade.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:
 1. In a method of additively coloring an alkali fluoride crystal selected from the class consisting of lithium fluoride, sodium fluoride, potassium fluoride, and rubidium fluoride, the improvement which comprises selecting an alkali metal fluoride crystal consisting essentially of between 0 and 5 ppmA of OH⁻, between 0 and 50 ppmA of one or more alkaline earth cations, between 0 and 50 ppmA non-fluorine halide anions, between 0 and 10 ppmA of SiF₆ ⁼, between 0 and 30 ppmA of other ions, and said alkali metal fluoride.
 2. In a method of additively coloring a sodium fluoride crystal, the improvement comprising selecting said sodium fluoride crystal consisting essentially of between 0 and 2 ppmA of OH⁻, between 0 and 50 ppmA of one or more alkaline earth cations, between 0 and 50 ppmA non-fluorine halide anions, between 0 and 10 ppmA of SiF₆ ⁼, between 0 and 30 ppmA of other ions, and sodium fluoride.
 3. In a method of additively coloring a sodium fluoride crystal, the improvement which comprises selecting said sodium fluoride crystal consisting essentially of between 0 and 0.5 ppmA of OH⁻, between 0 and 50 ppmA of one or more alkaline earth cations, between 0 and 50 ppmA non-fluorine halide anions, between 0 and 10 ppmA of SiF₆ ⁼, between 0 and 30 ppmA of other ions and sodium fluoride.
 4. A method of purifying an alkali metal fluoride crystal selected from the class consisting of lithium fluoride, sodium fluoride, potassium fluoride, and rubidium fluoride of ions detrimental to coloration by additive and electrolytic techniques which comprises:a. the step of converting said alkali metal fluoride crystal to alkali metal hydrofluoride which comprises reacting said alkali metal fluoride crystal with an aqueous solution of hydrogen fluoride at a temperature from about 25° C to about 90° C, whereby a mixture of alkali metal monohydrofluoride and alkali metal polyhydrofluorides is formed; b. the step of crystallizing, in a period of time from 10 to 12 hours, said mixture of alkali metal monohydrofluoride and of alkali metal polyhydrofluorides, whereby numerous crystals are produced; c. the step of regenerating in an inert and non-contaminating atmosphere alkali metal fluoride from crystals of said mixture of alkali metal monohydrofluoride and alkali metal polyhydrofluorides which comprises:1. removing water and unreacted hydrogen fluoride from said crystals;
 2. expelling, if necessary, reacted hydrogen fluoride from said alkali metal polyhydrofluorides of said mixture sufficient to obtain a mixture having at least 95% of alkali metal monohydrofluoride;
 3. heating said crystals to the temperature at which a pressure of 760 torr of hydrogen fluoride is evolved from said crystals;
 4. maintaining said temperature for at least 4 hours;5. heating, in not less than 6 hours, said crystals to a temperature of about one half of the melting point in degrees Celsius of said crystals;
 6. maintaining said temperature for about 1 hour;
 7. heating, in not less than 2 hours, said crystals to the melting point of said crystals; and
 8. maintaining said temperature for at least 2 hours, thereby completely forming molten alkali metal fluoride and eliminating all hydrogen fluoride; d. the step of crystallizing said molten alkali metal fluoride is not less than 8 hours into a single crystal.
 5. The method of claim 4 wherein said temperature of step (a) is from 80° C to 90° C.
 6. The method of claim 5 wherein said crystallization step (d) comprises:1. reducing in not less than 6 hours said temperature to the temperature at which crystallization begins; and
 2. withdrawing, as the temperature is reduced to room temperature in not less than 4 hours, 75% to 90% of said alkali metal fluoride into a single crystal boule by the Czochralski method.
 7. The method of claim 6 which further comprises:e. the step of removing, from each end of said alkali metal fluoride crystal produced in step (d), 5% of the length of said crystal; f. the step of melting said alkali metal fluoride crystal; and g. the step of recrystallizing in not less than 8 hours said alkali metal fluoride crystal by the Czochralski method.
 8. The method of claim 7 wherein said alkali metal hydrofluoride crystals are a mixture of alkali metal monohydrofluoride and alkali metal polyhydrofluorides consisting of less than 95% of alkali metal monohydrofluoride and step (c) comprises:1. heating, in not less than 2 hours, said crystals to a temperature from about 80° C to about 90° C;
 2. maintaining said temperature for at least 1 hour, thereby eliminating water;
 3. heating, in not less than 3 hours, to a temperature from 100° C to 3/4 of the temperature in degrees Celsius at which hydrogen fluoride starts to evolve from said crystals;
 4. maintain said temperature for at least 1 hour;
 5. heating, in not less than 5 hours, said crystals to the temperature at which said crystals again begin to evolve hydrogen fluoride;
 6. maintaining said temperature for 2 hours;
 7. repeating steps (5) and (6) until reaching the temperature at which alkali metal monohydrofluoride evolves a pressure of 760 torr of hydrogen fluoride;
 8. maintaining said temperature for at least 6 hours;9. heating, in not less than 12 hours, said crystals to the temperature of about one half of the melting point in degrees Celsius of said crystals;
 10. maintaining said temperature for about 1 hour;
 11. heating, in not less than 4 hours, said crystals to the melting point of said crystals; and12. maintaining said temperature for at least 2 hours; thereby completely forming molten alkali metal fluoride and eliminating all hydrogen fluoride.
 9. The method of claim 7 wherein said alkali metal hydrofluoride crystals are a mixture of alkali metal monohydrofluoride and alkali metal polyhydrofluorides consisting of at least 95% of alkali metal monohydrofluoride and step (c) comprises:1. heating, in not less than 5 hours, said crystals to a temperature from 100° C to three fourths of the temperature in degrees Celsius at which alkali metal monohydrofluoride begins to evolve hydrogen fluoride, thereby expelling water and unreacted hydrogen fluoride;
 2. heating, in not less than 5 hours, to the temperature at which alkali metal monohydrofluoride evolves a pressure of 760 torr of hydrogen fluoride;
 3. maintaining said temperature for at least 6 hours;
 4. heating, in not less than 12 hours, said crystals temperature of about one half the melting point in degrees Celsius of said crystals;5. maintaining said temperature for about 1 hour;
 6. heating, in not less than 4 hours, said crystals to the melting point of said crystals; and
 7. maintaining said temperature for at least 2 hours, thereby completely forming molten alkali metal fluoride and eliminating all hydrogen fluoride.
 10. In a method of additively coloring an alkali metal fluoride crystal selected from the class consisting of lithium fluoride, sodium fluoride, and potassium fluoride, improvement which comprises initially purifying said crystal according to the method of claim
 8. 11. In a method of additively coloring a sodium fluoride crystal, the improvement which comprises initially purifying said crystal according to the method of claim
 8. 12. In a method of additively coloring an alkali metal fluoride crystal selected from the class consisting of lithium fluoride, sodium fluoride, potassium fluoride, and rubidium fluoride, the improvement which comprises initially purifying said crystal according to the method of claim
 9. 13. In a method of additively coloring an alkali metal fluoride crystal selected from the class consisting of lithium fluoride, sodium fluoride, and potassium fluoride, improvement which comprises initially purifying said crystal according to the method of claim
 9. 14. In a method of additively coloring a sodium fluoride crystal, the improvement which comprises initially purifiying said crystal according to the method of claim
 9. 